In radio frequency (RF) applications, higher and higher cut-off frequency (Ft) of RF transistor is required. Silicon bipolar junction transistor (BJT) and especially SiGeC heterojunction bipolar transistor (SiGeC HBT) are the best options of high Ft devices. As SiGe is compatible with silicon process, SiGe HBT has already become one of the mainstream ultra high frequency devices.
In conventional BiCMOS process, parasitic vertical PNP bipolar transistor collector is picked up by sinker in active region to the heavily doped collector buried layer or well under STI. This approach is determined by the nature of such vertical device. The disadvantage of such structure is too big device size and high collector resistance. STI between sinker active to intrinsic collector is necessary for such structure, which further set limit for device size reduction.